The present disclosure relates in general to computer networking, and in particular to techniques for facilitating link redundancy in Layer 2 networks.
Virtual Switch Redundancy Protocol (VSRP) is a network protocol that allows for failover between redundant Layer 2 and/or Layer 2/3 devices (referred to herein as switches) in a loop-free Layer 2 network. In particular, VSRP enables a group of switches to act in concert as a single, virtual switch. One switch in the group (the VSRP master switch) can operate in a master mode in which the switch actively forwards data traffic on all ports connected to the Layer 2 network. Other switches in the group (the VSRP backup switches) can operate in a backup mode or master confirm mode in which the switches block data traffic on all ports connected to the Layer 2 network. The determination of which switch should act as the master and which switches should act as backups can be based on priority values that are derived from various criteria.
When the VSRP master switch experiences a failure or otherwise becomes unavailable, one of the VSRP backup switches can determine that it has the highest priority value and can automatically transition from backup/master confirm mode to master mode, thereby taking over switching functionality from the failed master. The features of VSRP are discussed in greater detail in the following U.S. patents and patent applications, the entire contents of which are incorporated herein by reference for all purposes: U.S. Pat. No. 7,209,435, issued Apr. 24, 2007 and entitled “SYSTEM AND METHOD FOR PROVIDING NETWORK ROUTE REDUNDANCY ACROSS LAYER 2 DEVICES;” U.S. Pat. No. 7,558,195, issued Jul. 7, 2009 and entitled “SYSTEM AND METHOD FOR PROVIDING NETWORK ROUTE REDUNDANCY ACROSS LAYER 2 DEVICES;” and U.S. patent application Ser. No. 12/477,069, filed Dec. 3, 2009 and entitled “SYSTEM AND METHOD FOR PROVIDING NETWORK ROUTE REDUNDANCY ACROSS LAYER 2 DEVICES”
One shortcoming with current implementations of VSRP is that redundancy is generally limited to the switch level. As a result, VSRP cannot adequately address failures that may occur at the network link level. Consider the network topology depicted in FIG. 1. As shown, a VSRP master switch 102 and a VSRP backup switch 104 (which are configured to act in concert as a VSRP virtual switch 106) are communicatively coupled with VSRP-aware switches 108, 110 via links 112, 114, 116, 118 and with a wide area network (WAN) 120. Per conventional VSRP functionality, VSRP master switch 102 is configured to forward data traffic on ports 122, 124 connected to links 112, 114 and VSRP backup switch 104 is configured to block data traffic on ports 126, 128 connected to links 116, 118. If VSRP master switch 102 fails, VSRP backup switch 104 can transition to master mode and begin forwarding data traffic on ports 126, 128, thereby taking over switching functionality from 102.
However, if a failure occurs with respect to one or more links 112, 114, 116, 118, a situation can occur where VSRP switches 102 and 104 cannot properly forward all data traffic. For example, consider a situation where link 114 fails, but where the priority value of switch 102 remains higher than the priority value of switch 104 (thus preventing switch 104 from transitioning to master mode). In this case, VSRP switch 102 will continue operating in master mode, but will not have a working connection to VSRP-aware switch 110. Accordingly, data communication with VSRP-aware switch 110 will be lost.